Lay down a coating of paint in one color. This color is the one that will show through the cracks in the other color. If, for example, you want a blue item with green cracks, apply the green paint now.
Allow the paint to dry completely. Read the instructions for the paint to determine how long this takes. It may be up to a full 24 hours or as little as overnight.
Paint the crackle medium over your first layer of paint. Only paint it on in one direction, which should be the way you want the cracks to flow. For example, if you want vertical cracks, apply the medium in strokes from top to bottom.
Let the crackle medium dry for approximately one hour, unless the manufacturer's instructions dictate a different length of time.
Apply the top coat of paint over the crackle medium with a paintbrush. This time, paint in the opposite direction as you did when you applied the crackle medium. Only go over each spot once, as the crackling begins immediately and going over the spot again ruins the effect.